In the past, batteries (also known as "dumb" cells) provided an unpredictable source of power, since typically, a user of a device powered by a battery had no reliable advance warning that a battery was about to run out of operable capacity. There was no indication of how much time was left so that a user could, for example, save the data currently being worked on or locate an alternate power source prior to complete discharge of the battery. As a result, a number of electronic products have been specifically designed to use the product's circuitry in an attempt to evaluate the battery's state of charge to determine when to begin back-up operations prior to the battery running out of capacity. This is done, for example, by measuring the terminal voltage of the battery, and, upon reaching a certain lower voltage limit, executing certain end-of-discharge operations, such as providing any necessary alarms or executing save-to-disk ("STD") operations.
This lower voltage limit is generally referred to as the end-of-discharge voltage (EODV) and is typically a constant, set according to the remaining capacity necessary for such end-of-discharge operations and the requirements for safe, efficient use of a cell. However, the discharge profile varies according to cell chemistry. Therefore, if a product's circuitry is designed to begin EODV save-to-disk operations at a set voltage for a certain type of cell, and a different cell with a different discharge curve is utilized to power the product, the EODV save-to-disk operations may be executed too soon resulting in a waste of capacity or, worse yet, too late resulting in loss of data.
An example of two cell chemistries which have different discharge curves are graphite based lithium-ion cells and coke-based lithium-ion cells. A typical graphite based lithium-ion cell can have a discharge profile as sown in FIG. 1. Various cell chemistries have certain lower limits for which they may be safely discharged to without adversely affecting the battery performance. For example, graphite based lithium-ion cells should not be discharged below 2.7 v. Coke-based lithium-ion cells, on the other hand, may be safely discharged to 2.5 v without adversely affecting performance. A typical coke-based lithium-ion cell can have a discharge profile as shown in FIG. 2. The discharge profile for coke-based lithium-ion cells gradually drops (FIG. 2), while the discharge profile for graphite based lithium-ion cells drops rapidly at the end of capacity (FIG. 1). Accordingly, the voltage near the end of capacity in graphite based lithium-ion cells is considerably higher than the coke-based lithium-ion cells. Therefore, if a product's circuitry is designed to begin EODV save-to-disk operations at a set voltage, e.g., 3.0 v for a coke-based lithium-ion cell (which would leave approximately 150 mAh capacity), and a graphite based lithium-ion cell is substituted therefor, when the terminal voltage reaches the preset voltage of 3.0 v, there is not enough capacity (e.g., only 40 mAh) remaining in the battery for such EODV save-to-disk operations and data may be lost. The same problems associated with the coke-based lithium-ion cells and graphite-based lithium ion cells may be recognized in other cell chemistries (e.g., Li-polymer, NiMH, and the like)
Accordingly, there is a need in the art to provide a way to allow a product, which utilizes terminal voltage measurements to trigger EODV operations, to begin such necessary EODV operations at the appropriate point in the discharge curve regardless of the actual terminal voltage.